The present invention relates to data link control method and system for an ISDN (Integrated Services Digital Network) and, more particularly, to a control method and system for multiplexing a call in the form of an originating call or incoming call by a communication control processor (CCP) connected to the ISDN through subscribers' lines to a data link connection of a D or B channel.
Hitherto, in a digital switching network, in the case where a host computer (hereinafter, referred to as a CPU) connected to a communication control processor (hereinafter, referred to as a CCP) connected to the switching network through a plurality of subscribers' lines and a packet terminal connected to the switching network through one subscriber's line communicate, upon originating a call, the CCP selects, for instance, a subscriber's line in which the number of calls which have been set (call multiplicity) is smallest or the subscriber's line in which a queuing (queue length) of the packet to be transmitted is shortest and multiplexes a new call to the selected subscriber's line. On the other hand, according to JP-A-61-48260, there has been proposed a system in which means for counting a residual amount of data blocks (packets) which are being transmitted or are in the transmission waiting mode is provided for every line, the CCP predicts the time until the end of data transmission of each line on the basis of a data transfer rate of each line and the counted residual data amount, and distributes a transmission data block which has been newly generated to the line in which the predicted time is shortest.
In recent years, the ISDN (Integrated Services Digital Network) having at least one control channel (hereinafter, referred to as a D channel) and a plurality of information channels (hereinafter, referred to as B channels) has been being put into practical use as a multi-dimensional information communication service network which can cheaply transmit various media such as audio, image, digital data, and the like at a higher speed as compared with the existing telephone network or digital switching network. Although the D channel itself of the ISDN can be also used as an information channel, the control, such as selection of a transfer mode (line switching/packet switching) of the information channels, designation of the kind of information channel to be used and the channel number, and the like, can be also executed by using the D channel. The channel numbers i of the B channels are set to 1 to 2 in the basic interface of the ISDN, while they are set to 1 to 23 in the primary rate interface. On the other hand, in the ISDN, the D channel comprises a channel for transmission of a control signal (hereinafter, referred to as a D.sub.1 channel) and a channel for transmission of a packet (hereinafter, referred to as a D.sub.2 channel). The D.sub.2 channel can be used as one of the information channels.
In the ISDN, if the transfer mode of either the circuit switching or the packet switching is selected with respect to one information channel for an originating call/incoming call, the transfer mode of the above information channel cannot be changed to another mode for a period of time when such a call is set. Therefore, for instance, when calls to request switching of the packets are sequentially generated, if the space information channel in which the waiting time is shortest is sequentially assigned to the new call, all of the information channels will become occupied as channels for switching of the packets. Thus, there is a problem that a call which needs the circuit switching which occurs after that cannot be communicated.